Thermal release device



Aug. 11', 1936.

H. V. ERBEN THERMAL RELEASE DEVICE Filed March 3,v 1954 m m w rE n w. mVf VWXCM bww@ H H H Uu Patented Aug'. 11, 1936 UNITED STATES THERMALumass nEvrcE Henry V..Erben, Philadelphia, Pa., assigner to GeneralElectric Company, a corporation of New York Application March 3,

9 Claims.

My invention relates to thermal release devices for circuit breakers andhas for an object the provision of a simple, inexpensive thermal releasehaving improved operating characteristics.

My invention is particularly applicable to circuit breakers of the typewhich are automatically operated to the open circuit position inresponse to a predetermined overload current. Heretofore it has .beencommon practice to restrain a switch opening member in a predeterminedposition by a latching member secured to a bimetal thermal element.Since the switch opening member is strongly biased to the open circuitposition a considerable amount of friction must be overcome by thethermal element in unlatching the circuit breaker. By providing polishedmetal surfaces the friction is substantially decreased. However, thepolished surface is quickly damaged by undue temperature such forexample as is produced by the flow of overload current through thecircuit breaker.

In carrying out my invention inV one form thereof, I provide a thermalrelease comprising two upright bimetallic members connected together bymeans of a shunt formed of a homogeneous strip of high electricalconductivity material and a' latching member formed of steel or thelike. The heating of the steel member is minimized since substantiallyall of the current flows through the high conductivity material, suchfor example as copper, the resultant heating of the steel member beingpractically negligible. Besides securing improved operation of thethermal release a substantial economy is effected in manufacture sincethere is no waste of the bimetallic material, such for example asordinarily occurs when special forms of thermal elements are stampedfrom stock. In accordance with my invention, the bimetallic material canbe produced in strips and cut to desired lengths without anyr wastewhatsoever.

For a more complete understanding of my invention, reference should nowbe had to the draw-A ing wherein I have shown in perspective in IFig. 1a thermal release device constructed in accordance with my invention,while in Fig. 2 I have shown in perspective a modification of myinvention.

Referring to the drawing, I have shown my invention in one form asapplied to a circuit breaker I2 of the type described in a copendingapplication of Winfield A. Atwood, Serial No. 667,855, filed April 25,19,33, entitled Circuit breakers and assigned to the same assignee asthe present invention. In the present application only so much 1934,Serial No. 713,898

of the circuit breakerisshown as is necessary for the explanation of myinvention.

An over-centerv spring (not shown) is operatively connected to a pair ofswitch operating links I3 connected together by a cross-portion 5 i3d tooperate a movable contact I4 between openv and closed circuit positionswith respect to a sta.-` tionary contact I5. An opening spring I6normally under compression biases a tripping member I8 and a downwardlyextending projection I8a 10 carried thereby in a direction to separatethe contacts. Normally, however, the tripping mem-A ber I8, providedwith a latching member 20, is held against movement by reason of theengagement of the latching member 20 with a steel latching 15- member orbar 2|, secured to two rectangular bimetallic thermal elements 22 and 24formed of strip material. Between the latching member or bar 2| and theupper or free ends 25 and 26 of the thermal elements there is provided acrossbar 28 formed of high electrical conductivity material, such forexample as copper. The copper crossbar 28 and the steel latching member2I are securely fastened tothe bimetallic elements 22 and 24 as byrivets 2i! and 30 formed of steel or 2'5 yMonel metal, while the lowerends of the bimetallic elements 22 and 24 are xedly secured to a pair ofrugged stationary terminal lugs 32 and 34 as by steel rivets 35 and 36.

In order to minimize the friction between the 30 latching member 20andthe steel latching bar 2|,v the surfaces 20a and Ia are highlypolished. These polished surfaces are protected from overheating sincesubstantially all of the current flows through the high conductivitymaterial 28, the current flow causing a minimum amount of heating due tothe low resistance of the material V28 Referring to the bimetallicmembers 22 and 24, it will be understood that the current flows' fromthe terminal 32 through the bimetallic elementV 24, and divides at itsupper end so that substantially all of the current flows through thehigh conductivity material 28 to the bimetallic element 22 and thence bya flexible conductor 40 to the switch contacts I4 and I5. 'I'hebimetallic elements of relatively high resistance material are heated inproportion to the magnitude oi.' the current. The bimetallic elements 22and 24 are arranged so that they bend in a counter-clockwise direction,as viewed in Fig. l, in response to an increase in temperature due tothe unequal expansion of the metals forming the bimetallic elements, aswill be understood by those skilled in the art. Any suitable metalshaving satisfactory electrical and heat resistance characteristics andhaving temperature coeilicients of expansion which are suicientlydissimilar may be used, such as nickel-steel and nickel-copper alloys.

As each bimetallic element is heated by the flow of a predeterminedcurrent it bends in a counter-clockwise direction, thus carrying thelatching bar 2| and the crossbar 28 away from the latching member 20 andreleasing it. Since all of the bimetal extends in a vertical direction,as shown substantially every portion of it is eiiiciently used toproduce deflection of the latchf ing bar 2|.

As soon as the latching member 20 is released the vcompression springvi6 rotates the downwardly extending projection |8a into engagement withthe cross-portion 13a to operate the movable contact I4 to the opencircuit position.

In U-shaped bimetallic elements the cross member formed of bimetallicmaterial and connecting the two vertical portions serves no usefulpurpose in the deflection of the element, but tends to twist thevertical portions so as to inhibit the deilection of the element.Furthermore, it has been found that during overload conditions thecurrent distribution through the U-shaped bi- .metallic elements is notuniform, the current tending to concentrate along the inner edges of thebimetallic elements. The .reason for the nonuniform now of current isthat the rivets securing the lower ends of the U-shaped elements totheir stationary terminal lugs produce the greatest contact pressure inthe portion of the material directly adjacent the rivets. The currentpath of low resistance is, therefore, the shortest distance between therivets, this current path of course being along the inner edge of theU-shaped ele ments.

In accordance with my invention, the fastening means for the bimetalelements are arranged so that the path of lowest electrical resistanceis through the central portion of the bimetallic elements and thecrossbar. Although any suitable securing means can be employed, forexample spot Welding, I have shown the rivets 30 and 35 of thebimetallic element 24 located in the center of the element. Similarlythe rivets 29 and 36 of the bimetallic element 22 are located in thecenter of the element. Consequently, the respective rivets 29, 30, 35,and 36 produce a greater contact pressure between the portion of theelements directly adjacent the rivets so that the current entering abimetallic element through one riveted fastening means flows by way oi'the lowest resistance/path `to the other riveted or welded fasteningmeans.

ySince the riveted fastening means are located in.

the centers of the elements 22 and 24 and the crossbar 28, there is notendency for the current to divide unevenly or to concentrate along theedges of the elements because the lowest resistance paths are alongstraight lines extendingbetween rivets and through the central portionof the bimetallic elements and the crossbar.- In actual operation of myinvention, the bimetal elements were uniformly heated, although underthe same overload conditions the U-shaped bimetal elements of the typeheretofore used were burned by the concentration of current along theinner edges of the elements. It has further been found that the timerequired for my thermal elements to produce a given deflection for agiven amount of current flowing through them is substantially less thanwith the U-shaped or Y-shaped thermal elements.

In 'case a magnetic trip is to be provided for the circuit breaker, acurrent responsive coil 45 can be mounted adjacent the latching member2| and the crossbar 28 so as to produce an attractive effort on thesteel latching member 2| in a direction to release the latching member20, the coil being connected across a shunt 48 connected in circuit withthe circuit breaker contacts. For applications of my invention tothermal releases having a low current rating, for example 15 amperes, Iprefer to use only a steel crossbar connecting the upper 'ends of thebimetallic elements. As shown in Fig. 2, a pair of bimetallic elements50 and 5| supported from the terminals 52 and 53 have their upper endsconnected together by a steel crossbar 54, riveted connections beingused, a latching portion 54a being polished to minimize friction. Forlow current ratings the steel crossbar 54 does not become heatedsumciently to injure the polished surface. The steel crossbar 54 can beused as the armature of a magnetic trip' in the same manner as describedin connection with the coil 45 of Fig. 1.

It will be understood that the current flows from the lug 53, the rivet56 which secures the bimetallic element 5| to the lug, through thebimetallic element 5|, through the rivet 51 which secures the upper endof the element 5| to the crossbar 54, thence through the crossbar 54 tothe rivet 58 which secures the other end of the crossbar 54 to thebimetallic element 50, and through the bimetallic element 50 and therivet 59 which secures the fixed end of the bimetal element 50 to thestationary lug 52. As I have stated in connection with Fig. 1, therespective rivets 56 and 59, inclusive, produce a greater contactpressure between the portion of the conductors directly adjacent therivets so that the lowest resistance path is between the rivets andsince the rivets are located in the center of the bimetallic elementsthere is no tendency for current to divide unevenly or to concentratealong the edges of the bimetal elements.

While I have shown a particular embodiment of my invention, it will beunderstood, of course, that I do not wish to be limited thereto sincemany modifications may be made, and I, therefore, contemplate by theappended claims to cover any such modifications as fall within the truespirit and scope of my invention.

IWhat I claim as new and desire to secure by Letters Patent of theUnited States is:

1. A current responsive thermal release device comprising, a pluralityof stationary terminal lugs, a bimetallic thermal element for each lug,means iixedly securing at one end said thermal elements to theirrespective terminal lugs, a homogeneous crossbar formed of highelectrical conductivity material, means connecting said crossbar to thefree ends of said elements to form a'. current path between saidelements, and a metallic latching member formed of low electricalconductivity material supported from said elements in electricalconducting relation with said elements and said crossbar, the heating ofsaid latching member incident to current ilow being minimized by saidhigh conductivity crossbar.

2. A current responsive thermal release device comprising, a pluralityof bimetallic thermal elements, means iixedly securing corresponding 70tween said elements having a lower electrical 75 resistance than theelectrical resistance of said elements, and a latching member ofrelatively low electrical conductivity supported by said elments andoperable by the deflection of said elements to and from a latchingposition, said low conductivity material preventing undue heating ofsaid latching member.

3. A current responsive thermal release device ior a circuit breakerhaving a tripping member one end of which is provided with a polishedlatching surface comprising, a plurality of bimetallic thermal elements,means iixedly mounting corresponding ends of said elements, ahomogeneous crossbar formed of high electrical conducting material,means securing said cross-bar to the opposite free ends of saidbimetallic elements to form a low resistance current path between saidelements, a metallic latching member provided with a polished surfacefor engaging said polished surface of said tripping member, meansmounting said latching member on said crossbar in electrical conductingrelation therewith for movement with said bimetallic elements, saidpolished surfaces being located intermediate the ends of said free endsof said bimetallic elements, said high conductivity material shuntingsaid latching member and thereby minimizing heating of said polishedsurfaces.

4. A current responsive thermal release device for a circuit breakerhaving a tripping member a portion of which is provided with a polishedlatching surface comprising, a pair of bimetallic thermal elements,means iixedly mounting corresponding ends of said elements, ahomogeneous crossbar formed of high electrical conductivity material, amagnetizable latching bar formed of low electrical conductivitymaterial, means securing said cross-bar in intimate electricalengagement with the opposite free ends of said bimetallic elements toform a Icurrent path between said eleinents having a lower electricalresistance than the' resistance of said elements, said means alsosecuring said latching bar against said crossbar, said latching barbeing provided with a polished surface intermediate said bimetallicelements for engagement with said polished latching surface of saidtripping member the heating of said surfaces due to the flow of currentthrough said magnetizable bar being minimized by said high conductivitycrossbar, and means responsive to current flow through said circuitbreaker for producing a magnetic attractive force on said magnetizablebar to release said tripping member irrespective ot said bimetallicelements.

5. A current responsive thermal release device comprising, a pair ofstationary terminal lugs spaced apart one from the other, a pair ofthermal elements formed of strip bimetallic material, means fixedlysecuring at one end thereof a bimetallic element to said lugs, ahomogeneous crossbar formed of high electrical conductivity material, a.steel latching member having a relatively low electrical conductivity,means seeming the ends of said crossbar and said latching membertogether in intimate electrical engagement, said means also securingsaid crossbar in intimate electrical engagement with the opposite freeends of said thermal elements to form a low resistance current pathbetween said free ends of said thermal elements whereby the heating ofsaid latching member due to current flow is minimized.

6. A current responsive thermal release device 5 comprising, a pluralityof stationary terminal lugs, a bimetallic thermal element for each lug,a rivet for iixedly securing at one end` a thermall element to itsterminal lug, a crossbar formed of electrically conductive material,rivets for coninecting said crossbar to the free ends of said elementsto form a current path between said elements, said rivets being locatedmidway between the edges of said elements so that the current path ofminimum resistance lies directly between said rivets and through thecenter portion of said bimetal element whereby an even currentdistribution through said bimetal elements is insured.

7. A current responsive thermal release device comprising, a pair ofstationary terminal lugs, a pair of bimetallic thermal elements, saidelements being provided with apertures adjacent their ends and locatedmidway between the edges of said elements, fastening means extendingthrough said apertures for fixedly securing the respective correspondingends of said bimetal elements to said lugs so as tc produce a maximumpressure of said elements against said lugs directly adjacent saidaperture, a crossbar formed of electrically conductive material,fastening means extending through the apertures in the iree ends of saidelements for securing said crossbar thereto and for producing a pressurefor urging said free ends of said elements into intimate relation withsaid crossbar, the maximum pressure being applied to the elementsadjacent said apertures.

8. A current responsive thermal release device comprising, a pair ofstationary terminal lugs, a pair of bimetallic. thermal elements, acrossbar formed of electrically conductive material, fastening means forxedly securing corresponding ends of said bimetal elements to said lugsand for securing said crossbar to said opposite ends of said bimetalelements so that the lowest resistance current path is located throughthe centrai portions of said bimetallic elements and said bar whereby auniform heating of said bimetal elements is assured -under allconditions.

9. A current responsive thermal release device comprising, a pluralityof stationary terminal lugs, a bimetal thermal element for each lug,fastening means flxedly securing at one end said thermal .elements totheir respective terminal lugs, a crossbar formed of high electricallyconductive material, means connecting said crossbar to the free ends ofsaid elements to form a current path between said elements, and alatching member formed of low electrically conductive material supportedfrom said elements whereby the heat of said latching bar incident tocurrent flow is minimized by said crossbar, said fastening means forsaid elements forming a current path of minimum resistance extendingthrough the central portions of said bimetal elements and said crossbar.

HENRY V. ERBEN.

